1. Field of the Invention
The present invention relates to searching for small thermal targets, such as a human swimmer in open water, and particularly in the context of a search and rescue, port security, or homeland defense operation.
2. Background of the Related Art
It is often necessary to locate small thermal targets, such as mammals, vehicles, periscopes, or watercraft, amidst a comparatively vast expanse of open land or water. Humans are the most common subject of such searches, such as during a search and rescue (“SAR”) operation following an aircraft or watercraft accident in open water. Other open-water searches are sometimes performed for law enforcement patrols and counter-terrorism efforts, such as in the enforcement of “open-ocean homeland security.” In these searches, it may be necessary to detect a small target, such as a periscope or other indicator of a submersed object. Land searches may include, for example, the search for humans or animals of interest in the wilderness. Even automobiles, watercraft, or aircraft may be the subject of a search. In the context of open water searches for humans, the most basic method of detecting a human target involves visually scanning an area of water, using the naked eye, such as from an aircraft. The effective maximum sweep width with this approach under optimal visibility conditions is 0.1 nautical miles (NM), or about 600 feet, according to the United States National Search and Rescue Supplement to the International Aeronautical Maritime Search and Rescue Manual, published in May of 2000 by the National Search and Rescue Committee (“NSARC”). Visual inspection is extremely difficult and factors such as fatigue can play a considerable role in the effectiveness of such a search. Therefore, a variety of systems and methods have been implemented to assist or supplant naked-eye searches.
Radar is one example of a system used extensively in open-water searches. However, human targets are generally too small to detect with radar. Both Forward-Looking Airborne Radar (“FLAR”) and Side-Looking Airborne Radar (“SLAR”) are better suited to detecting ships and other large-scale objects than humans. The smallest object deemed accessible by either method is a 4-person life raft, according to the NASARC.
Another search system and method involves Forward Looking Infrared (“FLIR”). FLIR is currently the most effective commercially-available means of swimmer detection. However, it is still greatly limited in its small-target search capability. At a wide field of view (“WFOV”), the spatial resolution on the screen viewed by the human user is too coarse to resolve small targets. When the imager is used with a narrow field of view (“NFOV”), only a small search area can be viewed at any one time. Since a human searcher requires time to recognize a search target in the field of view, the rate of progression for a search is quite slow. This is often referred to as the “soda straw” effect. The NSARC-recommended sweep width for a FLIR system when looking for a swimmer is 0.3 NM, assuming a Douglas Sea Scale (“DSS”) of no more than 1. For reference, a DSS value of 1 corresponds to “smooth” seas with wave heights of no more than 1 foot. Above a DSS value of 1, the recommended sweep width drops to 0. In stronger seas, the waves can momentarily obscure the swimmer from direct view and, thereby, the detection opportunity may be missed by conventional methods. Since the field of view of the imager is rather narrow, the infrared (“IR”) imager used in FLIR must typically raster back and forth to cover a wide area of interest.